IGCSE Chemistry Moles Calculations Explained Step by Step

You can balance the equation and you know what a mole is, but the moment a question says "calculate the mass of..." you freeze. Moles calculations feel like a pile of different formulas, so it is hard to know which one to reach for. The good news is that almost every IGCSE Chemistry moles question runs on the same four-step method, and once you see it, the topic stops being scary.

This guide is written for Cambridge IGCSE Chemistry (0620). Mole calculations mostly sit in the Stoichiometry section and are heavily tested on the Extended (Supplement) papers, so check your own syllabus document for exactly which parts apply to your tier and exam year.

What is a mole in IGCSE Chemistry?

A mole is just a counting unit for particles, like a "dozen" but much bigger. One mole of anything contains 6.02 × 10²³ particles (the Avogadro constant). You never count particles directly, so you use mass to count them instead.

The link between mass and moles is the relative formula mass, Mr, which is just the sum of the relative atomic masses (Ar) in a formula. The Mr in grams is the mass of one mole.

For example, NaOH: 23 + 16 + 1 = 40, so one mole of NaOH has a mass of 40 g.

The three formulas you actually need

Most of the topic comes down to three relationships:

Two details that cost marks every year: the 24 dm³ figure is for gases at room temperature and pressure (r.t.p.), and volumes in cm³ must be divided by 1000 to get dm³ before you use the concentration formula.

Tip: Write "moles = ..." at the top of your working before you substitute numbers. It forces you to say which of the three relationships you are using and earns method marks even if the arithmetic slips.

The four-step method for any moles question

Use this order every time.

1. 1Write the balanced equation. The coefficients (big numbers in front) are your mole ratio.
2. 2Convert what you are given into moles using one of the three formulas above.
3. 3Use the ratio from the equation to get the moles of the substance you want.
4. 4Convert moles into what the question asks for: mass, volume or concentration.

Think of moles as a bridge. You always walk from "given" to moles, across the equation, then from moles to "wanted". Let us practise.

Worked example 1: mass to moles

Question: How many moles are in 4.0 g of sodium hydroxide, NaOH?

1. 1Mr of NaOH = 23 + 16 + 1 = 40
2. 2moles = 4.0 ÷ 40 = 0.10 mol

This is the shortest possible moles question, and it is the first step of nearly every longer one.

Worked example 2: reacting masses and gas volumes

Question: 6.0 g of magnesium reacts with excess hydrochloric acid. What volume of hydrogen gas is produced at r.t.p.? (Ar: Mg = 24)

Mg + 2HCl → MgCl₂ + H₂

1. 1Moles of Mg = 6.0 ÷ 24 = 0.25 mol
2. 2Ratio Mg : H₂ is 1 : 1, so moles of H₂ = 0.25 mol
3. 3Volume = 0.25 × 24 = 6.0 dm³

"Excess" tells you the acid will not run out, so the magnesium decides how much gas you get.

Worked example 3: concentration and titration

Question: 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide is neutralised by 20.0 cm³ of sulfuric acid. Find the concentration of the acid.

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

1. 1Moles of NaOH = 0.100 × (25.0 ÷ 1000) = 0.00250 mol
2. 2Ratio NaOH : H₂SO₄ is 2 : 1, so moles of H₂SO₄ = 0.00250 ÷ 2 = 0.00125 mol
3. 3Concentration = moles ÷ volume = 0.00125 ÷ (20.0 ÷ 1000) = 0.0625 mol/dm³

The classic trap here is forgetting the 2 : 1 ratio. If your answer comes out suspiciously double or half what you expected, check the ratio first.

Tip: If a question gives concentration in g/dm³, convert it to mol/dm³ by dividing by the Mr. If it asks for g/dm³, multiply your mol/dm³ answer by the Mr.

Limiting reactants: which one runs out?

When both reactants are given as amounts, one will run out first. That one is the limiting reactant, and it controls how much product you can make.

Question: 2.4 g of magnesium burns in 3.2 g of oxygen. What mass of magnesium oxide forms? (Ar: Mg = 24, O = 16)

2Mg + O₂ → 2MgO

1. 1Moles of Mg = 2.4 ÷ 24 = 0.10 mol
2. 2Moles of O₂ = 3.2 ÷ 32 = 0.10 mol
3. 3The equation needs 2 mol Mg for every 1 mol O₂, so 0.10 mol Mg needs only 0.05 mol O₂. Oxygen is in excess, so magnesium is limiting.
4. 4Moles of MgO = 0.10 mol (ratio 2 : 2). Mr of MgO = 40, so mass = 0.10 × 40 = 4.0 g

The habit to build: compare the moles you have to the moles the equation needs, not the masses.

Percentage yield

Reactions rarely give you everything the equation predicts. Percentage yield compares what you actually got with the maximum possible.

percentage yield = (actual yield ÷ theoretical yield) × 100

Question: 10.0 g of calcium carbonate is heated and 4.2 g of calcium oxide is collected. What is the percentage yield?

CaCO₃ → CaO + CO₂

1. 1Mr of CaCO₃ = 100, so moles = 10.0 ÷ 100 = 0.100 mol
2. 2Ratio is 1 : 1, so theoretical moles of CaO = 0.100 mol
3. 3Mr of CaO = 56, so theoretical mass = 0.100 × 56 = 5.6 g
4. 4Percentage yield = (4.2 ÷ 5.6) × 100 = 75%

Common mistakes to avoid

• Using an unbalanced equation. The ratio will be wrong and everything after it too.
• Mixing up cm³ and dm³. Divide cm³ by 1000, every time.
• Using the Mr of the wrong substance. Underline the substance you are converting before you work out its mass.
• Rounding too early. Keep extra digits until the final line, then give an answer to a sensible number of significant figures.
• Forgetting units. Marks are often given for "mol", "g" or "dm³" with the final answer.

How to practise so it sticks

Reading worked examples feels productive, but it is not the same as being able to do them. Research on learning shows that actively recalling information beats re-reading it (Roediger & Karpicke, 2006), and that explaining material in your own words leads to better understanding than studying it again (Fiorella & Mayer, 2013).

Here is a simple routine:

1. 1Cover the worked example and try the question cold.
2. 2Explain each step out loud as if teaching a friend: why divide here, why use this ratio?
3. 3Check against the method above and note exactly which step went wrong.
4. 4Repeat the same type of question two days later, then again a week later.

Our guide on why teaching a topic is the fastest way to learn it goes deeper on step 2, and how to use topical past papers shows how to turn mark scheme feedback into progress. If moles is one topic among many, slot it into a revision schedule with our IGCSE study plan guide.

Quick recap

• A mole counts particles, and you reach it through mass, gas volume or concentration.
• Always follow the bridge: given → moles → ratio → wanted.
• Check the balanced equation, the units and the ratio before you submit your answer.

If you want to test whether you can really explain moles, try teaching it to an AI pupil on TeachVerse, which only knows what you tell it and shows your gaps when it answers exam-style questions. You can start free here.